Certain acronyms may be used herein and are defined below:
APC—Automatic Power Control
CMOS—Complementary metal-oxide-semiconductor
cw—Continuous wave
DFB—Distributed Feedback
ECL—Emitter-coupled logic
FET—Field-effect transistor
Gb/s, Gbps—Gigabits per second (data rate)
Gigabit—109 bits
GHz—Giga Hertz, 109 cycles per second (frequency)
LD—Laser diode
MQW—Multi-quantum well
MHz—Mega Hertz, 106 cycles per second
NIC—Network interface card
nm—Nanometer, 10−9 Meters
PCB—Printed circuit board
PRBS—Pseudo-random bit sequence
s—Complex frequency, jω+α
SPDT—Single-pole, double-throw
TEC—Thermo-electric cooler
TOPIX—Transparent Optical Protocol Independent Switching
TTL—Transistor-transistor logic
In some optical communication system architectures, several laser transmitters from different NICs on the same domain share a common optical fiber. Consequently, only one laser at a time is allowed to transmit, and the light from the others must be turned off. In principle, several means including external modulation and switching of the light can be used. However, cost and speed requirements call for direct control of the laser diode current. Thus, during the intervals when a given laser is turned off, no bias or modulation currents are allowed into the laser. The functional requirement of allowing only one laser to transmit by rapidly turning selected lasers on and off is also needed in other shared-media passive optical networks (PON) that are under development for local telecommunications networks.
On the other hand, a system requirement is that the (average) optical power from the laser be held constant while it is transmitting. Factors that tend to cause variation in this power are temperature variation (with characteristic times varying from seconds to days), and long-term aging (with characteristic times on the order of months to years). Lasers for long-distance telecommunications typically have internally stabilized temperature using thermoelectric coolers. However, in many applications the cost, package size, and power consumption required by temperature stabilization preclude its use.
Other systems compensate for laser power changes with temperature variation by controlling the laser bias current. Typically, both temperature and aging changes are compensated by simple closed-loop feedback, which monitors the laser average output power and regulates the bias current into the laser. The optical sensor that monitors the laser output power is an internal photo-diode that is incorporated in the laser package.
During burst-mode operation, when the laser may be deliberately turned off for long intervals, long-term average optical power monitoring may no longer be suitable for regulating the bias current. The system should monitor the fast time-average modulated power only during the ON burst, and use this signal to provide current feedback.
The laser diodes can be driven by current-source circuitry, which is usually provided by specialized laser driver chips that allow for separate fast data modulation outputs and slower offset bias currents. The driver bias current circuitry generally responds relatively slowly so the feedback voltage that controls the bias cannot be turned on and off to follow the burst pattern. For some known driver chips, the total output current can be enabled or disabled separately and quickly by an external gating signal.
Maxim Integrated Products has issued an Application Note (HFDN-14.0) that describes a modification to the Evaluation Board for their MAX3867 Laser Driver that purports to accomplish burst-mode APC. The feedback signal from the monitor photo-diode is input directly to the driver chip, and its internal circuitry and filtering are described in a high-level fashion. The photo-current signal appears to be amplified by a proportional trans-impedance feedback amplifier. With the burst-mode modification, the filter capacitor is switched either directly to the driver, or through a high-impedance buffer amplifier. This circuit does not function for data bursts less than 16 μs. However, certain applications require shorter data bursts, such as about 5 μs.
It would, therefore, be desirable to provide a system in which a slowly varying bias feedback signal can be applied to the driver, while it (and the laser) is rapidly gated on and off by a signal, such as an ENABLE/DISABLE signal, that regulates the burst mode operation.